An example of the typical modern approach to generating high purity oxygen plus co-product crude argon by cryogenic distillation is presented by R. E. Latimer in "Distillation of Air", Chemical Engineering Progress, Volume 63 No. 2, February 1967, published by the American Institute of Chemical Engineering. Other examples can be found in U.S. Pat. Nos. 4,433,990, 3,751,934, and 3,729,943.
The distillation column configuration normally encountered comprises a lower column and upper column in heat exchange relationship, i.e., a "dual pressure" column, and an auxiliary crude argon column which directly connects to an intermediate height of the upper column. Functionally, the lower column is a rectifying column which receives the cooled and cleaned supply air at its base, pressurized to about 6 ATA. The overhead rectification product N.sub.2 condenses against boiling oxygen bottom product of the upper or low pressure column, which has a bottom pressure of about 1.5 ATA. The LP column has three sections which accomplish different functions. The bottom section strips argon from the oxygen so as to achieve product purity. Above this section the column is divided into two sections. One section receives (directly or indirectly) the partially evaporated kettle liquid from the HP rectifier bottom as feed, and distills or removes the nitrogen overhead from that liquid, leaving a fairly pure oxygen-argon liquid mixture which drops into the argon stripping section. The second top section is the argon rectifying section (sidearm), in which the fraction of reboil entering it from the common connection point of the three sections is rectified to crude argon overhead, plus a fairly pure oxygen-argon liquid mixture which also drops into the argon stripping section. Thus vapor transiting up through the argon stripping section splits into two streams, one continuing up the N.sub.2 removal section and the other going up (reboiling) the argon rectification section. Similarly liquid transiting downward through the latter two sections combines at the common connecting point, and all the combined liquid flow continues refluxing downward through the argon stripping section.
The overhead of the argon stripping section is normally cooled (refluxed) by indirectly exchanging latent heat with at least part of the kettle liquid, and the resulting at least partially evaporated kettle liquid is fed to the N.sub.2 removal section. The N.sub.2 removal section is normally refluxed by direct injection of liquid N.sub.2 (LN.sub.2) from the HP rectifier overhead product into the top of the N.sub.2 section.
The problems which limit the amount of crude argon possible to recover with the above configuration are as follows. The relative reboil rates up the two top sections of the LP column are the primary determinants of the argon recovery. About 10% of the argon appears as impurity in the oxygen product, and the remainder is split between the overhead products of the N.sub.2 removal section and the argon rectification section in rough proportion to the amounts of reboil up each section. The combined reboil entering those two sections is a fixed amount, namely that going up the argon stripping section. The N.sub.2 removal section has a minimum reboil requirement--the amount necessary for it to reach its feed introduction point without pinching out. The more oxygen present on the feed plate or tray, the lower that reboil requirement. This is why designs which totally evaporate kettle liquid are more efficient than those which only partially evaporate it for argon rectifier reflux. The totally evaporated feed has a higher O.sub.2 content than does the vapor associated with the partially evaporated feed, and therefore is properly introduced at a lower tray height of the N.sub.2 removal section.
Since there is a minimum N.sub.2 removal section reboil requirement, and a fixed total amount of reboil available, there is correspondingly a maximum amount of reboil available for the argon rectifier. In order to increase argon recovery, it is necessary to either decrease the N.sub.2 removal section reboil to below the minimum amount otherwise required when totally evaporated kettle liquid is fed, or to increase argon rectifier reboil to above its corresponding maximum allowed amount, or preferably do both simultaneously. This is not possible with present designs.
In one prior art reference, U.S. Pat. Nos. 3,729,943, some increase in argon recovery is achieved by increasing the reboil through the argon stripping section only. This is done by locating a latent heat exchanger at the common connection point between the three sections of the LP column, and evaporating LN.sub.2 or LOX in that exchanger. By increasing reboil through the argon stripping section, a higher O.sub.2 purity is obtained (assuming the same number of trays/countercurrent contact stages/theoretical plates). Thus up to 10% less argon exits with the O.sub.2 product. However, the saved argon is still split in the same proportions between the N.sub.2 removal section and the argon rectification section, and hence only part of it is actually recovered. This is because the reboil rates through those two sections are unchanged. Even though the latent heat exchanger is physically located in the bottom of the argon rectifier, all the trays of the argon rectifier are above the latent heat exchanger, and hence the latent heat exchanger causes no added reboil through any of the countercurrent contact part of the argon rectifier.
In the above disclosure, when LN.sub.2 is evaporated at the bottom of the argon rectifier, that vapor may be work expanded to produce the required process refrigeration. That vapor is at a substantially lower pressure than the HP rectifier overhead vapor, e.g., at 4.5 ATA vice 6 ATA. Accordingly a proportionately larger amount must be expanded to produce a given refrigeration requirement. In modern "LOXBOIL" plants this will have an adverse impact on O.sub.2 recovery. LOXBOIL plants are those in which the product oxygen is evaporated by latent heat exchange against either partially or totally condensing air vice against condensing HP rectifier overhead gas (typically 99+% purity N.sub.2). This substantially increases the delivery pressure of the product oxygen, but it decreases the amount of LN.sub.2 available to reflux the N.sub.2 removal section and the HP rectifier, and thus decreases the ability to rectify the O.sub.2 out of those two overhead products. LOXBOIL plants can recover about 97% of the oxygen as product provided only 8 to 10% of the feed gas is work expanded, but any additional work expansion causes a reduction in achievable O.sub.2 recovery. Thus the prior art disclosure, in a LOXBOIL context, provides some additional argon recovery but at the expense of reduced product oxygen recovery, due to the higher N.sub.2 flow required for refrigeration expansion.
There is another reason why attempts to increase argon recovery have an adverse impact on O.sub.2 recovery of LOXBOIL plants, even in the absence of the LN.sub.2 evaporator at the bottom of the argon rectifier as disclosed in the prior art. As argon recovery increases (and holding argon purity constant), there are two different and additive effects which both require increases in the reboil rate up the argon rectification section. First, greater mass flow out the top (overhead product) at a fixed column L/V will require a linearly proportional increase in V (reboil). More importantly, however, as the argon recovery increases, the argon concentration at the common connecting point between the three LP column sections decreases. For most modern LOXBOIL plants having an argon recovery of about 60%, that concentration is about 7 or 8% argon. For zero recovery, it must increase to about 17%, to force all the argon up the N.sub.2 removal section. If full recovery were possible, it would decrease to about 4%. As argon recovery is increased, and the connecting point concentration correspondingly decreases, the feed vapor to the argon rectification section is located lower on the equilibrium line of the McCabe-Thiele diagram, and hence a decreased L/V is actually required, thus further increasing both the reboil and reflux requirement for a given argon recovery.
With two requirements to increase the reboil and reflux, more kettle liquid must be evaporated to supply the reflux; at the limit, all is evaporated. This, however, shifts, the N.sub.2 removal section feed point substantially down the equilibrium line, to the extent that the LN.sub.2 reflux available to the N.sub.2 removal section can no longer rectify sufficient oxygen out of the overhead nitrogen, and hence O.sub.2 recovery suffers.
From the foregoing it can be seen that the need which exists in this technical field, and one objective of the present invention, is to provide a means for increasing argon recovery in dual pressure cryogenic air separation plants without decreasing the oxygen recovery, purity, or delivery pressure, or increasing the input energy requirements. Specifically, the objectives are to increase the argon rectifier reboil rate and preferably also decrease the N.sub.2 removal section reboil rate relative to what is possible now, without decreasing O.sub.2 recovery, and with only one added heat exchanger; to provide additional refrigeration without decreasing the reflux available to the N.sub.2 removal section overhead; to recover a greater fraction of the increased argon obtained from increased reboil through the argon stripper via LN.sub.2 depressurization; and other objectives.
These objectives apply to "gas only" plants, and also to plants having part or all of their product in liquid phase. Also, in many plants which recover both high purity oxygen and crude argon, it is also desired to recover substantial quantities of coproduct nitrogen at a pressure above the LP column pressure, either as gas or liquid. The above objectives apply to those plants also.
Copending U.S. Pat. No. 4,670,031 issued to Donald C. Erickson on June 2, 1987 discloses two methods of achieving the above objectives, both of which involve an intermediate reflux condenser associated with the argon rectifier or sidearm. In one method, intermediate height liquid from the N.sub.2 removal section of the LP column is supplied to the intermediate reflux condenser, and the resulting vapor is returned to the N.sub.2 removal section as intermediate reboil therefor. In the second method, liquid N.sub.2 (LN.sub.2) is evaporated in the intermediate reflux condenser and subsequently work expanded. The present invention is a further extension of that second method, in recognition of the facts that the same advantageous objectives can be achieved without the work expansion step, and even further advantages are obtainable by incorporating refrigeration N.sub.2 companding, and/or TC LOXBOIL with liquid air split, and/or companding of the TC LOXBOIL air, and/or kettle liquid distillation for overhead refluxing of the argon rectifier.
Refrigeration N.sub.2 companding is disclosed in copending application No. 8831230 filed on July 2, 1986 by Donald C. Erickson. Companded TC LOXBOIL plus liquid air split is disclosed in copending application 853461 filed on Apr. 18, 1986 by Donald C. Erickson.
Refluxing of the argon rectifier overhead via kettle liquid distillation, and resultingly feeding two vapor streams of differing O.sub.2 content to the N.sub.2 removal section, is disclosed in copending application 893045 filed Aug. 1, 1986 by Donald C. Erickson.
Additional prior art patents pertinent to this invention include U.S. Pat. Nos. 2,411,680, 2,672,031, 2,779,174, 3,605,423, 4,099,945, and 4,133,662. Additional general background art is in the technical article, "Production of Large Quantities of Oxygen by an Improved Two-Column Process", Moscow IIR (XIV International Congress of Refrigeration), Sept. 1975, by Martin Streich and Josef Dworschak.